Fabrication and Characterization of ZnO Nanorods Based Intrinsic White Light Emitting Diodes (LEDs) Nargis Bano

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Linköping Studies in Science and Technology Dissertation No. 1401

Fabrication and Characterization of ZnO Nanorods Based Intrinsic

White Light Emitting Diodes (LEDs)

Nargis Bano

Physical Electronics and Nanotechnology Group Department of Science and Technology (ITN)

Linköpings Universitet, Campus Norrköping SE-601 74 Norrköping

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noorbwn@gmail.com

ISBN: 978-91-7393-054-3 ISSN 0345-7524

Printed by Liu-Tryck, Linköping University, Linköping, Sweden

Linköping Studies in Science and Technology Dissertation No. 1401

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Fabrication and Characterization of ZnO Nanorods Based Intrinsic White

Light Emitting Diodes

Nargis Bano

Department of Science and Technology, Linköping University Norrköping Sweden

Abstract:

ZnO material based hetero-junctions are a potential candidate for the design and realization of intrinsic white light emitting devices (WLEDs) due to several advantages over the nitride based material system. During the last few years the lack of a reliable and reproducible p-type doping in ZnO material with sufficiently high conductivity and carrier concentration has initiated an alternative approach to grow n-ZnO nanorods (NRs) on other p-type inorganic and organic substrates. This thesis deals with ZnO NRs-hetero-junctions based intrinsic WLEDs grown on p-SiC, n-SiC and p-type polymers. The NRs were grown by the low temperature aqueous chemical growth (ACG) and the high temperature vapor liquid solid (VLS) method. The structural, electrical and optical properties of these WLEDs were investigated and analyzed by means of scanning electron microscope (SEM), current voltage (I-V), photoluminescence (PL), cathodoluminescence (CL), electroluminescence (EL) and deep level transient spectroscopy (DLTS). Room temperature (RT) PL spectra of ZnO typically exhibit one sharp UV peak and possibly one or two broad deep level emissions (DLE) due to deep level defects in the bandgap. For obtaining detailed information about the physical origin, growth dependence of optically active defects and their spatial distribution, especially to study the re-absorption of the UV in hetero-junction WLEDs structure depth resolved CL spectroscopy, is performed. At room temperature the CL intensity of the DLE band is increased with the increase of the electron beam penetration depth due to the increase of the defect concentration at the ZnO NRs/substrate interface. The intensity ratio of the DLE to the UV emission, which is very useful in exploring the origin of the deep level emission and the distribution of the recombination centers, is monitored. It was found that the deep centers are distributed exponentially along the ZnO NRs and that there are more deep defects at the root of ZnO NRs compared to the upper part. The RT-EL spectra of WLEDs illustrate emission band covering the whole visible range from 420 nm and up to 800 nm. The white-light components are distinguished using a Gaussian function and the components were found to be violet, blue, green, orange and red emission lines. The origin of these emission lines was further identified. Color coordinates measurement of the WLEDs reveals that the emitted light has a white impression. The color rendering index (CRI) and the correlated color temperature (CCT) of the fabricated WLEDs were calculated to be 80-92 and 3300-4200 K, respectively. Keywords: Zinc Oxide nanorods, White light emitting diode, Photoluminescence, Cathodoluminescence, Electroluminescence, Deep level transient spectroscopy (DLTS).

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iv List of publications included in the thesis

This thesis consists of an introductory text and the following papers:

I. Study of radiative defects using current-voltage characteristics in ZnO rods catalytically grown on 4H-p-SiC

N. Bano, I. Hussain, O. Nur, M. Willander, and P. Klason, Journal of Nanomaterials, Article ID 817201(2010).

II. Study of luminescent centers in ZnO nanorods catalytically grown on 4H-p-SiC N. Bano, I. Hussain, O. Nur, M. Willander, P. Klason and A. Henry, Semicond. Sci. Technol. 24, 125015 (2009).

III. Depth-resolved cathodoluminescence study of zinc oxide nanorods catalytically grown on p-type 4H-SiC

N. Bano, I. Hussain, O. Nur, M. Willander, Q. Wahab, A. Henry, H. S. Kwack and D. Le Si, Dang, Journal of Luminescence 130, 963–968 (2010).

IV. Study of Au/ZnO nanorods Schottky light-emitting diodes grown by the low- temperature aqueous chemical method

N. Bano, I. Hussain, O. Nur, M. Willander, H. S. Kwack and D. Le. Si Dang, Appl. Phys A. 100, 467–472 (2010).

V. ZnO-organic hybrid white light emitting diodes grown on flexible plastic using low temperature aqueous chemical method

N. Bano, S. Zaman, A. Zainelabdin, S. Hussain, I. Hussain, O. Nur, and M. Willander, J. Appl. Phy. 108, 043103 (2010).

VI. Study of intrinsic white light emission and its components from ZnO-nanorods/p-polymer hybrid junctions grown on glass substrates I. Hussain, N. Bano, S. Hussain, O. Nur and M. Willander, J. Mater Sci.46, 7437 (2011).

VII. Study of the distribution of radiative defects and reabsorption of the UV in ZnO nanorods-organic hybrid white LEDs

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I. Hussain, N. Bano, S. Hussain, Y. Soomro, O. Nur, and M. Willander, Materials 4, 1260-1270 (2011).

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vi Related Papers not included in the thesis

1. ZnO as an energy efficient material for white LEDs and UV- LEDs M. Willander, O. Nur, A. Zainelabdin, N. Bano. I. Hussain, S. Zaman, and M. Q. Israr, submitted (2011).

2. Intrinsic white light emission from zinc oxide nanorods heterojunctions on large area substrates Magnus Willander, O. Nur, S. Zaman, A. Zainelabdin, G. Amin, J. R. Sadaf, M. Q. Israr, N. Bano, I. Hussain, and N. H. Alvi, Proceedings of SPIE 7940, 79400A (2011).

3. Zinc Oxide nanorods/polymer hybrid heterojunctions for white light emitting diodes M. Willander, O. Nur, S. Zaman, A. Zainelabdin, N. Bano, and I. Hussain, J. Phys. D: Appl. Phys. 44, 224017 (2011).

4. Luminescence study of ZnO hybrid white LEDs grown on cheap/disposable

substrates by low temperature chemical growth

I. Hussain, N. Bano, M. Y. Soomro, O. Nur, and M. Willander Nanoscinece and Nanotechnology submitted (2011).

5. Inorganic-organic ZnO based heterostructures for lighting M. Willander, N. Bano, and O. Nur, ECS Transactions, 19 (12) 1-12 (2009).

6. Luminescence from zinc oxide nanostructures and polymers and their hybrid

devices

M. Willander , O. Nur, J. R. Sadaf, M.Q. Israr, S. Zaman, A. Zainelabdin, N. Bano and I. Hussain, Materials 3, 2643-2667 (2010).

7. Zinc oxide nanorod-based heterostructures on solid and soft substrates for

white-light-emitting diode applications M. Willander, O. Nur, N. Bano and K. Sultana, New Journal of Physics 11, 125020

(2009).

8. Different interfaces to crystalline ZnO nanorods and their applications M. Willander, M. H. Asif, S. Zaman, A. Zainelabdin, N. Bano, S. M. Al-Hilli, and O.

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9. Current-transport studies and traps extraction of hydrothermally grown ZnO nanotubes using gold Schottky diode G. Amin, I. Hussain, S. Zaman, N. Bano, O. Nur, and M. Willander, Phys. Status Solidi A 207, No. 3, 748–752 (2010).

10. Photonic nano-devices and coherent phenomena in some low dimensional systems M. Willander ,Yu. E. Lozovik, S. P. Merkulova, O. Nur, A. Wadeasa, P. Klason, B. Nargis, N. H. Alvi, and S. Kishwar, 214th ECS Meeting, Abstract #2034, © The Electrochemical Society.

11. Enhancement of zinc interstitials in ZnO nanotubes grown on glass substrate by the hydrothermal method M. Y. Soomro, I. Hussain, N. Bano, S. Hussain, O. Nur and M. Willander, Appl. Phys A submitted (2011).

12. Growth and characterization of ZnO nanotubes on disposable-flexible paper

substrates by low temperature chemical method M. Y. Soomro, I. Hussain, N. Bano, Jun Lu, O. Nur and M. Willander (manuscript).

13. Nanoscale elastic modulus of single horizontal ZnO nanorod using nano-indentation experiment M. Y. Soomro, I. Hussain, N. Bano, E. Broitman, O. Nur and M. Willander, Nanoscale Research Letters, submitted (2011).

14. ZnO nanorods based nanogenerator on flexible paper substrate M. Y. Soomro, I. Hussain, N. Bano, O. Nur and M. Willander, (manuscript).

15. Study of deep level defects in ZnO nanorods grown on p-GaN by low temperature chemical method

I. Hussain, N. Bano, M. Y. Soomro, M. Asghar, O. Nur and M. Willander, (manuscript).

16. Comparative study of ZnO nanorods based WLEDs grown on p-SiC by high and

low temperature growth methods

N. Bano, I. Hussain, M. Y. Soomro, O. Nur and M. Willander, (manuscript).

17. The Study of surface states in ZnO nanorods/p-GaN heterojunctions

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Acknowledgement

All praise goes to ALLAH, who created the universe and appointed the man as His vicegerent. I offer my humble thanks to ALLAH who blessed and enabled me to complete this dissertation in stipulated time frame. All the blessings to His Prophet Muhammad (peace be upon him), who is the source of guidance and knowledge for humanity.

Special appreciation goes to my supervisor Associate Prof. Omer Nour for his supervision and constant support. His invaluable help of constructive comments and suggestions throughout this work which have contributed to the success of this research.

I would like to warmly acknowledge my co-supervisor Prof. Magnus Willander for his guidance and input throughout the process of this research.

Not forgotten, my appreciation to the ex-research administrator Lise-Lotte Lönndahl Ragnar and our present group research administrator Ann-Christin Norén for their administrative help during my studies and research work.

I would like to express my deep and obedient appreciation gratitude to Dr. Anne. Henry, Dr. D. Le. Si Dang, Dr. H. S. Kwack, Dr. Peter Klason, Dr. Asghar Hashmi and Dr. Qamar ul Wahab and Sajjad Hussain for their endless support and co-operation in my research work.

I am really thankful to Higher Education Commission (HEC), government of Pakistan for partial financial help in during my research work. I am also very thankful to Dr. Atta ur Rehman ( Ex- Chairman HEC), Dr. Javeed Laghari (Chairman HEC), Muhammad Ashfaq, project manager (HEC), Dr. Sohail Naqvi, and Dr. Yasir Jameel for their cooperation and good wishes.

I am also thankful to all of my group members. Many thanks for your cooperation and nice company.

I offer my sincerest wishes and warmest thanks to Muhammad Yousuf Soomro and his family. They help me and give me a nice company. I am also thankful to my sincere friends Sobia Shakir, Aisha Rana and Mona Ijaz.

It is those who are near to us that must bear the full force of our own inadequacies, so I must appreciate my brothers and sisters Haji Abdul Quyam, Khazar Hayat, Abdul Qadeer, Abdul Zaheer, Muhammad Aqil, Haji Imtiaz Hussain, Hafiz Mumtaz Hussain, Faraz

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Hussain, Shehzad Hussain and my sister Zatoon Fatima, Humeera Hussain and Dr. Naghmana Batool they always have given me support and love. I really appreciate them from the bottom of my heart.

I would like to express my profound admiration and salute to my father Haji Muhammad Iqbal my mother Sikandra Bibi, my father in law Muhammad Hussain and my mother in law Sarwri Begum. I would essentially have not been able to achieve this noble goal without their help and kindness. I wish to express my appreciation for their love and affection on me in every aspect of life.

At last I am very thankful to my loving husband Dr. Ijaz Hussain Asghar and my sweet son Muhammad Mughees without their unconditional support and encouragements nothing was possible.

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energies in ZnO nanorods. ... 46 Figure 5.5: (b) The emission intensity (right axis) of the NBE, green band and red as well as the ratio of the NBE and the green bands (left axis) as a function of the penetration depth ... 47 Figure 5.6: Comparison of the CL emission spectra from ZnO NRs and a nearby area

without ZnO NRs (4H-SiC area). ... 47 Figure 5.7: Typical room temperature current-voltage characteristics for the Au/ZnO NRs Schottky diode ... 49 Figure 5.8: Room temperature EL spectra of Au/ZnO NRs Schottky diode. ... 49 Figure 5.9: (a) Depth dependent CL spectra of ZnO NRs at room temperature ... 51 Figure 5.9: (b) The emission intensity (right axis) of the NBE, and DLE as well as the ratio of NBE and DLE (left axis) as a function of penetration depth ... .51 Figure 5.10: Comparison of the CL emission spectra from ZnO NRs and 4H-SiC area.. .... 52 Figure 5.11: Typical room temperature I-V characteristics of the ZnO NRs/PFO hybrid

WLED and the inset show the semilog plot of I-V characteristics. ... 53 Figure 5.12: (a) Room temperature PL spectrum of the ZnO NRs grown by the same

parameters on plastic substrate with no PFO film.. ... 54 Figure 5.12: (b) Room temperature PL spectrum of the ZnO NRs/PFO hybrid structure and

the inset shows the room temperature PL spectrum of the PFO on PEDOT:PSS plastic ... 54 Figure 5.13:Room temperature EL spectrum and the Gaussian fitting of the PFO/ZnO

hybrid WLED. ... 56 Figure 5.14: Typical color coordinates characteristics of the ZnO-PFO hybrid WLEDs.. ... 56 Figure 5.15: Room temperature PL spectrum of the ZnO NRs/PFO hybrid structure and the

PL spectrum of the PFO on glass substrate. ... 58 Figure 5.16: Typical room temperature I-V characteristics of the ZnO

NRs/PFO/PEDOT:PSS heterojunction. ... 58 Figure 5.17: (a) Room temperature EL spectrum after a Gaussian fitting... 59 Figure 5.17: (b) The EL spectrum showing the whole visible range which is distinguished by a Gaussian fitting.. ... 59 Figure 5.17: (c) The chromaticity diagram of the HWLED... ... 60

Figure 5.18: Room temperature EL and PL spectra of HWLEDs with PL spectra of PFO (without ZnO). ... 61

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Figure 5.19: (a) Energy band diagram of ZnO NRs/PFO hybrid heterojunction showing the EL emissions from ZnO NRs.. ... 61 Figure 5.19: (b) Energy band diagram of ZnO NRs/PFO hybrid heterojunction under forward bias showing the band banding at the interface and the charge accumulation... 62 Figure 5.20: Depth dependent CL spectra of the hybrid LED at room temperature... ... 64

Figure 5.21: Room temperature CL spectrum of the hybrid LED after the Gaussian fittin .. 64 Figure 5.22: Comparison of the CL emission spectra from the hybrid LED and an area with PFO only and the inset shows a magnified CL emission spectra of PFO.. ... 65 Figure 5.23: The emission intensity (right axis) of the UV, violet and DBE as well as the ratio of the UV and the DBE (left axis) as a function of the penetration depth………..66

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List of Tables

Table 2.1: Basic physical properties of ZnO at RT [4, 5, 7-9]………8 Table 2.2: Electrical properties of ZnO and GaN at RT [4, 5, 10-13]……….9

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Chapter 1 Introduction

The history of light emitting diodes (LEDs) goes back to 1907 [1] when it was reported for the first time by Henry Joseph Round but gained practical importance only in 1962 [2] when Nick Holonyak invented the first red light emitting diode. Since then, the field of LEDs has attracted a huge amount of attention both from industry and academia. However, not until recently, 50 years later, the fruit of the first report has been harvested through commercialization of LEDs. These days LEDs of all colors with reasonable efficiency are available which opens the opportunity to use LEDs in areas beyond conventional signage and indicator applications. Now LEDs can be used in high-power applications thereby enabling the replacement conventional light sources. LEDs light sources, which are more compact, more durable and capable to change color in real time, are finding more applications in domestic, commercial and industrial environments. However, at the moment LEDs have high prices, and nonlinear optical and electrical characteristics, which are major barriers for widespread lighting applications.

Since the last decade zinc oxide (ZnO) is being considered a potential candidate for LEDs due to several potential advantages over GaN-based material system. ZnO is an II-VI semiconductor material with direct band gap ~ 3.37 eV and the room temperature exciton binding energy ~ 60 meV, the large exciton energy makes it possible to employ excitonic recombination process as a lasing mechanism [3]. The sudden enormous increase of the global interest in ZnO material is due to the fact that it has diverse range of nanostructures including nanorods (NRs), nanobelts and nanotubes which can be grown on any substrate without the need of lattice matching [4]. The growth of ZnO NRs can be achieved using different high and low temperature growth methods. The most usual high temperature techniques used are metal organic chemical vapor deposition (MOCVD) and vapor liquid solid (VLS) and the aqueous chemical growth (ACG) method is low temperature techniques [4]. ZnO nanostructures possess novel integrated nano-systems with excellent optoelectronic properties for LEDs because of their extremely small volume and modified light-matter interaction [5]. ZnO NRs are the most important nanostructures which offer additional advantages for light emission due to the increased junction area, enhanced polarization dependence of reflectivity, and improved carrier confinement in one dimensional

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nanostructure [4]. In addition ZnO NRs have extremely smooth ends which make them perfect mirror planes and vertical ZnO NRs are like natural waveguide cavities for making the emitted light to travel to the top of the device. At the same time, ZnO NRs can reduce the threshold of simulated emission which appeared due to the possible radial quantum confinement effects in two dimensions in which the finite size of nano-scale materials that confines the spatial distribution of the electrons gives rise to the quantized energy level, due to that ZnO NRs possess high density of states at the band edge [6-8]. The quantum confinement effect plays an important role and enhancs the oscillation strength of the excitons, which is favorable for radiative recombination of excitons at room temperature these properties open the way to fabricate ZnO NRs based LEDs [9].

ZnO NRs have large number of intrinsic and extrinsic deep-level defects that emit different colors of light including violet, blue, green, yellow, orange and red, i.e. all constituents of the white light [10,11]. This implies that ZnO NRs have potential of being used as intrinsic white light emitting diodes (WLEDs). In order to develop ZnO NRs based WLEDs, the most important issue is the fabrication of low-resistivity p-type ZnO. Because unintentionally undoped ZnO shows typically n-type properties, acceptors may be compensated by native defects such as zinc interstitial (Zni), oxygen vacancy (VO), or

background impurities such as hydrogen [6]. Thus the lack of a reliable and reproducible p-type doping in ZnO with sufficiently high carrier concentration has instigated an approach to grow ZnO nanostructures on other p-type substrates to realize ZnO-based p–n hetero-junctions. Moreover, in order to achieve high internal quantum efficiencies, free carriers need to be spatially confined. This requirement also led to the development of p-n hetero-junction LEDs consisting of different semiconductors.

This thesis presents some effectual ways to produce ZnO NRs based hetero-junctions based intrinsic WLEDs on p-SiC, n-SiC and p-type polymers by ACG and VLS methods and provide some beneficial results in aspects of their structural, electrical and optical properties, which builds experimental and theoretical foundation for much better understanding the deep level emission physics.

Typically ZnO NRs exhibit one sharp ultraviolet (UV) peak and possibly one or two deep level emissions (DLE) bands due to deep level defects within the band gap. The dominant emitted colors depend on the growth conditions and methods. This means that the emitted colors can be controlled and that the luminescence efficiency is directly or indirectly

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related to deep level defects [4]. Therefore the defects are investigated using deep level transient spectroscopy (DLTS) and current-voltage characteristics in paper I and II.

The performance of ZnO NRs based WLEDs depend upon the defect chemistry, the distribution of defects and the origin of specific emissions from ZnO NRs and the substrates, which are the subjects of recent theoretical and experimental studies. Therefore in paper III the detailed information about the origin of specific emission, growth dependence of the optically active defects and their spatial distribution the depth resolved cathodoluminescence spectroscopy are performed. The cathodoluminescence is a powerful technique for characterizing the optical properties of ZnO NRs because it has a resolution below the diffraction limit of light [12].

In paper IV we reported for the first time ZnO NRs based Schottky LEDs grown on n-SiC substrates by the ACG method. These days ZnO-organic hybrid LEDs becomes one of the most exciting research areas because hybrid materials promise good properties that may not easily be achieved from conventional materials such as combining the high flexibility of polymers with the structural, chemical and high functional stability of inorganic materials. Such a route may pave the way for the apprehension of large-area photonic devices that could serve as building blocks for the next generation of flexible display panels. In paper V we reported white light luminescence from ZnO-organic hybrid LEDs grown at 90 oC on flexible plastic substrates. This will provide lighting designers the ability to put flexible white light panels in places previously impossible or prohibitively expensive.

In paper VI we reported white-light luminescence from ZnO-organic hybrid LEDs grown on glass substrate. The configuration used for the LEDs consists of two-layers of polymers on glass with top ZnO nanorods (NRs). Electroluminescence spectra show the combination of emission bands arising from radiative recombination in polymer and ZnO NRs. The transitions causing these emissions are identified and discussed in terms of the energy band diagram of the hybrid junction and the influence of the transparent ITO ohmic contact on the emitted intensity is discussed.

The UV emission can be internally reabsorbed by the ZnO crystal itself within 1µm range this reabsorbed UV emission can excite defect states in the ZnO material resulting in an increase of the intensity of the DBE. Thus, it is possible that part of the UV emission may contribute to the enhancement of the DBE. Therefore in paper VII depth dependent CL spectra was examined to investigate the UV re-absorption and the detailed information of the

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luminescence and especially the spatial distribution of the defects that cause the UV and the wide visible emissions from ZnO nanorods-organic hybrid white LEDs.

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References:

[1] H. J. Round, Electrical World 19, 309 (1907).

[2] N. Holonyak, and S. F. Bevacqua, Appl. Phys. Lett. 1, 82 (1962).

[3] M. Willander, Y. E. Lozovik, Q. X. Zhao, O. Nur, Q. H. Hu and P. Klason, Proc. SPIE 6486, 648614 (2007).

[4] N. Bano, S. Zaman, A. Zainelabdin, S. Hussain, I. Hussain, O. Nur, and M. Willander, J. Appl. Phys. 108, 043103 (2010).

[5] R. Könenkamp, C. Robert and C. Schlegel, Appl. Phys. Lett. 85, 6004 (2004). [6] K. Kitamura, T. Yatsui, M. Ohtsu and G. C. Yi, Nanotechnology. 19, 175305 (2008). [7] M. Huang, S. Mao, H. Feick, Y. Wu, H. Yan, H. Kind, E. Weber, R. Russo and P.

Yang, Science. 292, 1897 (2001).

[8] Z. Chen, Z. Shan, S. Li, C. Liang and X. Mao, Journal of Crystal Growth. 265, 482 (2004).

[9] Z. Zhou, Y. Zhao and Z. Cai, Applied Surface Science. 256 4724 (2010).

[10] M. Willander, O. Nur, Q. X. Zhao, L. L. Yang, M. Lorenz, B. Q. Cao, J. Zúñiga Pérez, C Czekalla, G Zimmermann, and M. Grundmann, A. Bakin, A. Behrends, M. Al- Suleiman, A. El-Shaer, A. Che Mofor, B. Postels and A. Waag, N. Boukos and A. Travlos H. S. Kwack, J. Guinard and D. Le Si Dang, Nanotechnology 20, 332001 (2009).

[11] M. Willander, O. Nur, Jamil Rana Sadaf, Muhammad Israr Qadir, S. Zaman, A. Zainelabdin, N. Bano and I. Hussain, Materials, 3, 2643 (2010).

[12] Y. Choi, J. Kang, D. Hwang and S. Park, IEEE Transactions on Electron Devices. 57, 26 (2010).

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Chapter 2 Zinc oxide material

In the past few years zinc oxide (ZnO) has received much attention because it has a wide range of properties that depend on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity and wide-bandgap semiconductivity. Without much effort, it can be grown in many different nanoscale forms on any substrate, thus allowing various novel devices to be achieved. In this chapter we discuss the different properties of ZnO nanostructures.

2.1 Basic properties of ZnO

ZnO is a wide band gap semiconductor with a large number of attractive properties for electronics and optoelectronics devices. At room temperature the bandgap energy is 3.37 eV. ZnO belongs to II-VI semiconductor group and the native doping of the ZnO is n-type. It has large excitons binding energy 60 meV due to that the excitons in ZnO are thermally stable at the room temperature. This semiconductor has many favorable properties like high electron mobility, good transparency, strong room temperature luminescence, ect. Those properties are used in electronic applications of ZnO as thin-film transistors and light-emitting diodes [1-3].

2.2 Crystal structure of ZnO

At room temperature (RT) ZnO has a hexagonal wurtzite crystal structure with lattice parameters a=3.25 Å and c=5.12 Å. The ZnO structure consists of alternating Zn and O layers as shown in Fig. 2.1. Each oxygen anion is surrounded by four zinc cations at the corner of the tetrahedron, and vice versa. Although the entire unit cell of ZnO is neutral. The polarity observed in the ZnO crystal is due to the tetrahedral structure. ZnO has four common surfaces, the polar Zn (0001) and O (0001) terminated faces and the nano-polar (1120) and (1010) faces. The (1120) and (1010) surfaces contain equal numbers of Zn and O atoms. At relatively modest external hydrostatic pressures wurtzite ZnO can be transformed to the rocksalt (NaCl) like other II-VI semiconductors. As compared to the wurtzite structure zincblende structure has lower ionicity, this lead to the lower carrier scattering and higher doping efficiencies [5, 6].

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Figure 2.1: The hexagonal wurtzite structure of ZnO. The O-atoms are the large red spheres and the Zn-atoms are the small grey spheres [5, 6]

2.3 Physical properties of ZnO at RT

Some basic physical properties of ZnO at 300K are listed in the Table 2.1 [4, 5, 7, 8]. The thermal conductivity variation is due to crystal defects so some of the values in the table have uncertainty [9] and because of the problems of producing robust and reproducible p-type doping there is also uncertainty in the values of the hole mobility and effective mass.

Table 2.1: Basic physical properties of ZnO at RT [4, 5, 7-9]

Parameters Values

Lattice constant a = 0.32495 nm, c =0.52069 nm

Density (g cm-3) 5.606

Stable phase at RT Wurtzite

Melting point (oC) 1975

Thermal conductivity 0.13, 1-1.2

Static dielectric constant 8.656

Refractive index 2.008, 2.029

Bulk Young´s modulus, E (GPa) 111.2±4.7

Electron effective mass 0.23m0

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2.4 Electrical properties of ZnO as compared to GaN

ZnO has several advantages as compared to its chief competitor GaN. ZnO has a direct bandgap of 3.37 eV at RT and higher exciton binding energy 60 meV at RT as compared to GaN 25 meV. However, GaN has a higher electron Hall mobility at RT as compared to ZnO but ZnO has higher effective mass and larger optical phonon parameters. ZnO has a higher saturation velocity which is very important for high speed devices [4].

Table 2.2: Electrical properties of ZnO and GaN at RT [4, 5, 10-13]

Parameters ZnO GaN

Band gap energy (eV) 3.37 3.39

Electron mobility (cm2/Vs) ∼210 1000-1350

Hole mobility (cm2/Vs) ∼10 100-400

Exciton binding energy (meV) 60 21-25

2.5 Properties and device applications

ZnO displayed a wide range of useful properties as it has been recognized for a long time [14]. Due to the fact that ZnO is a semiconductor with a direct band gap of 3.44 eV (at low temperature) it captured most of the attention in recent years [15–17]. ZnO in principle enables optoelectronic applications in the blue and UV regions of the spectrum. A list of the properties of ZnO that distinguish it from other semiconductors or render it useful for applications includes:

2.5.1 Direct and wide band gap

The band gap of ZnO is 3.37 eV at room temperature and 3.44 eV at low temperatures but the respective values for wurtzite GaN are 3.50 eV and 3.44 eV [15, 18]. Due to these properties ZnO is an important material for light emitting diodes, laser diodes, photodetectors and optoelectronics in the blue/UV region [4, 5, 7, 19, 20]. Reports on p–n homojunctions have recently appeared in the literature [21-24], but stability and reproducibility have not been established.

2.5.2 Large exciton binding energy

Due to the large free-exciton binding energy (60 meV) [25, 26] an efficient excitonic emission in ZnO can persist at room temperature and higher [25, 26]. Due to this large free-exciton binding energy ZnO is most promising material for optical devices that are based on excitonic effects.

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10 2.5.3 Strong luminescence

ZnO is a strong luminescence material because it possesses a number of intrinsic and extrinsic radiative defect levels which emit light in a wide range within the visible region [27] due to this property ZnO is also a suitable material for phosphor applications. Due to n-type conductivity, ZnO is one of the best material for applications in vacuum fluorescent displays and field emission displays. The origin of the luminescence mechanism and the luminescence center are still under discussion, without any clear evidence, being frequently attributed to oxygen vacancies or zinc interstitials [28].

2.6 Optical properties of ZnO

Due to a number of intrinsic and extrinsic radiative defect levels ZnO emit light in a wide range within the visible region [27]. There are a variety of experimental techniques available for the study of optical transitions in ZnO such as reflection, photoreflection, transmission, optical absorption, photoluminescence (PL), cathodoluminescence (CL), spectroscopic ellipsometry, and calorimetric spectroscopy [29].

The room temperature photoluminescence spectrum of ZnO is characterized by the two main peaks, a sharp UV peak centered on 380 nm and another broad deep band emission that lies literally between 400 nm and up to close to 800 nm. Figure 2.3 shows a typical PL spectrum of ZnO nanorods at room-temperature [30].

Due to the radiative defects different wavelength emissions from ZnO have been observed. The deep broad band emission from the ZnO exhibited violet, blue, green, yellow and orange-red color emissions [29], i.e. it covers the whole visible region.

Figure 2.2: PL spectrum for ZnO NRs measured using an Ar-laser operating at 150 W at room temperature [30].

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2.7 ZnO nanostructures

ZnO has a rich family of nanostructures such as nanowires, nanorods, nanocages nanocombs, nanotubes, nanosprings and nanorings. The different kinds of nanostructures can be obtained by changing growth condition. By the fabrication of white light emitting diode we use ZnO nanorods as shown in figure 2.3.

Due to their potential use in nanotechnology ZnO nanostructures has received broad attention. Light emitting diodes (LEDs), field effect transistors (FET), different sensors and other devices using ZnO nanostructures have been demonstrated. Due to the intrinsic and extrinsic defects ZnO has the possibility of white light emission. Nanostructured LEDs are of great interest due to white light emission from ZnO [38, 39].

Figure 2.3: SEM image of ZnO nanorods grown by the ACG method.

2.8 Native point defects in ZnO

Native or intrinsic defects are imperfections in the crystal lattice that involve only the constituent elements [31]. The enormous interest in semiconductors is that both electrical and optical properties of semiconductors can be modified by incorporation of small amounts of impurities or other defects. We can introduce defects during the growth process and also through annealing or ion implantation.

2.9 Classification of defects

Defects can be classified into

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12 2. Point defects

3. Complex defects 1. Line defects

Line defects involve rows of atoms, e.g. dislocations. 2. Point defects

Point defects involve isolated atoms in localized regions of the host crystal. 3. Complex defects

Complex defects are a composition of more than one point defect. Intrinsic and Extrinsic defects

If the defects involve foreign atoms, i.e. impurities, they are referred to as extrinsic defects but the defects that only consist of host atoms are called intrinsic defects [5].

Classification of point defects

Point defects are further classified as follows: Vacancy

Missing atoms at regular lattice positions called vacancy. The notation is VA. This defect is

always intrinsic. In ZnO both zinc (VZn) and oxygen (Vo) vacancies can be formed.

Substitutional

In case of substitutional a host atom A is replaced with an atom C, and it is denoted by CA.

Interstitial

Interstitials (extra atoms occupying interstices in the lattice) and antisites (a Zn atom occupying an O lattice site or vice versa). An intrinsic interstitial defect is often called self-interstitial, whereas the extrinsic interstitial defect is referred to as an interstitial impurity. Antisite

Antisite is a special kind of substitutional defect in this a host atom A is replaced by another host atom B. The anisite is denoted by the same notation as for substitutional defects. These defects are intrinsic and in case of ZnO both Zn and O exist.

2.10 Exciton

A bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force is called an exciton. It is a quasiparticle that exists in insulators, semiconductors and some liquids. An exciton can move through the crystal and transport energy but cannot change the charge since it is electrically neutral. An exciton forms when a photon is absorbed by a semiconductor. This excites an electron from the valence band into the conduction band. Exciton has been studied in two cases [32, 33].

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2.10.1 Frenkel exiton

In materials with a small dielectric constant, the Coulomb interaction between an electron and a hole may be strong and the excitons thus tend to be small, of the same order as the size of the unit cell. The Frenkel excitons, named after Yakov Frenkel has strong electron-hole interaction, i.e. the excitons are tightly bound. They have a typical binding energy on the order of 0.1 to 1 eV. Frenkel excitons are realized in alkalihalide crystals and in organic molecular crystals composed of aromatic molecules [32, 33].

2.10.2 Wannier excitons

The dielectric constant is generally large in semiconductors. Consequently, the Coulomb interaction between electrons and holes tends to reduced by electric field screening. So in most of the semiconductors the excitons are weakly bond. These excitons are called Wannier exciton, and the binding energy is usually much less than the hydrogen atom, typically on the order of 0.01 eV [32, 33, 34].

2.11 Recombination

Recombination is a process in which the electrons combine with holes in one or multiple steps and eventually disappear. The energy difference between the initial and final state of the electron is released and can leads to one or more possible classification of the recombination processes, e.g. radiative recombination, Auger recombination or Shockley-Read-Hall recombination (SRH) [35, 36].

2.11.1 Radiative recombination (Band to band recombination)

In radiative recombination the electron in the conduction band directly combines with the hole in the valance band and releases a photon therefore it is also called band to band recombination. The light produced from a light emitting diode (LED) is the most obvious example of radiative recombination in a semiconductor device [35, 36].

2.11.2 Auger recombination

In Auger recombination the electron and a hole recombine, but no electromagnetic radiation is emitted, and the excess energy and momentum of the recombining electron is given up to another electron. Auger recombination is mostly important at high temperatures and in heavily doped materials.

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14

Recombination through defects or Shockley-Read-Hall (SRH) recombination occurs when an electron trapped by defect level within the bandgap. One can envision this process either as a two-step transition of an electron from the conduction band to the valence band [35, 36].

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References:

[1] K. Ogata et al., Journal of Crystal Growth 251, 623 (2003). [2] T. Makino et al., Appl. Phys. Lett. 77, 975 (2000).

[3] T. Makino et al., Appl. Phys. Lette. 78, 1237 (2001). [4] D. C. Look, Mater. Sci. Eng., B 80, 383 (2001).

[5] C. Jagadish and S. J. Pearton Zinc Oxide Bulk, Thin Film and Nanostructures, Elseviser Ltd., (2006).

[6] A. Ashrafi and C. Jagadish, J. Appl. Phys. 102, 071101 (2007).

[7] Ü. Özgür, Ya. L. Alivov, C. Liu, A. Teke, M. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, and H. Morkoc, J. Appl. Phys, 98, 041301 (2005).

[8] T. Olorunyolemi, A. Birnoim, Y. Carmel, O. Wilson, I. Lioyd, S. Smith, and R. Campbell, J. Am. Cera. Socie, 85, 1249 (2002).

[9] D. I. Florescu, L. G. Mourokh, F. H. Pollak, D. C. Look, G. Cantwell, and X. Li, J. Appl.Phys, 91, 890 (2002).

[10] U. Ozgur et al., J. Appl. Phys. 98, 1 (2005).

[11] National Compound Semiconductor Roadmap, US Office of Naval Research, www. Onr.navy.mil/sci_tech/31/312/ncsr/default.asp, (2008).

[12] M. Rubin, N. Newman, J. Chan, T. Fu, and J. Ross, Appl. Phys. Lett. 64, 64 (1994). [13] D. C. Look et al., Solid State Commun. 105, 399 (1998).

[14] M. E. Brown (ed) ZnO-Rediscovered (New York: The New Jersey Zinc Company) (1957).

[15] A. Mang, K. Reimann and Rübenacke, Solid State Commun. 94, 251 (1995).

[16] D. C. Reynolds, D. C. Look, B. Jogai, C. W. Litton, G. Cantwell and W. C. Harsch, Phys. Rev. B 60, 2340 (1999).

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16

[17] Y. Chen, D. M. Bagnall, K. T. Koh, K. T. Park, K. Hiraga, Z. Q. Zhu and T. Yao, J. Appl. Phys. 84, 3912 (1998).

[18] O. Madelung (ed) Semiconductors—Basic Data 2nd Revised Edn (Berlin: Springer) (1996).

[19] S. B. Ogale Thin Films and Heterostructures for Oxide Electronics (New York: S pringer) (2005).

[20] N. H. Nickel and E. Terukov (ed) Zinc Oxide-A Material for Micro- and Optoelectronic Applications (Netherlands: Springer), (2005).

[21] T. Sukazaki A et al Nature Mater. 4, 42 (2005).

[22] R. Y. Ryu, W. J. Kim and H. W. White, J. Cryst. Growth 219, 419 (2000).

[23] L. J. Mandalapu, Z. Yang, F. X. Xiu, D. T. Zhao and J. L. Liu, Appl. Phys. Lett. 88, 092103 (2006).

[24] S. Chu, M. Olmedo, Z. Yang, J. Kong and J. L. Liu, Appl. Phys. Lett. 93, 181106 (2008).

[25] D. C. Reynolds, D. C. Look and B. Jogai, Solid State Commun. 99, 873 (1996).

[26] D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen and T. Goto, Appl. Phys. Lett. 70, 2230 (1997).

[27] C. Klingshirn, Chem. Phys. Chem. 8, 782 (2007).

[28] S. Shionoya and W. H. Yen (ed) Phosphor Handbook By Phosphor Research Society (Boca Raton, FL: CRC Press) (1997).

[29] M. Willander, O. Nur, Q. X. Zhao, L. L. Yang, M. Lorenz, B. Q. Cao, J. Zúñiga Pérez, C. Czekalla, G. Zimmermann, and M. Grundmann, A. Bakin, A. Behrends, M. Al- Suleiman, A. El-Shaer, A. Che Mofor, B. Postels and A. Waag, N. Boukos and A. Travlos, H. S. Kwack, J. Guinard and D. Le Si Dang, Nanotechnology 20, 332001 (2009).

[30] N. Bano, I. Hussain, O. Nur, M. Willander, P. Klason and A. Henry 2009 Semicond. Sci. Technol. 24, 125015 (2009).

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[31] M. Lannoo and J. Bourgoin 1981 Point Defects in Semiconductors I: Theoretical Aspects (Berlin: Springer), M. Lannoo and J. Bourgoin Point Defects in Semiconductors II: Experimental Aspects (Berlin: Springer) (1983).

[32] D. C. Reynolds and T. C. Collins, Excitons their properties and uses, Academic, (1981). [33] R. S. Knox, Theory of Excitons, Solid State Physics, Suppl. 5, (1963).

[34] Wannier, Gregory "The Structure of Electronic Excitation Levels in Insulating Crystals". Physical Review 52, 191 (1937).

[35] G. P. Agrawal and N. K. Dutta Long Wavelength Semiconductor Lasers (John Wiley and Sons, New York) (1986).

[36] R. K. Ahrenkiel “Minority-carrier lifetime in III-V semiconductors” in Minority Carriers in III-V semiconductors: Physics and Application edited by R. K. Ahrenkiel and M. S. Lundstrom, Semiconductors and Semimetals 39, 40 (Academic Press, San Diego) (1993).

[37] http://ecee.colorado.edu/~bart/book/recomb.htm

[38] M. Willander et al., Proc. SPIE 6486, 648614 (2007).

[39] Q. X. Zhao, L. L Yang, M. Willander, B. E. Sernelius and P. O. Holtz, J. Appl. Phys. 104, 073526 (2008).

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Chapter 3 Growth and characterization of ZnO nanorods based LEDs

ZnO nanostructures can be grown by using different high as well as low temperature (< 100 oC) growth approaches. In our work, we used the vapor-liquid-solid (VLS) and the aqueous chemical growth (ACG) methods. Due to the lack of a reliable and reproducible p-type doping in ZnO material we grow n-ZnO on other p-type substrates. In this chapter we discuss the growth methods of the ZnO nanorods on different substrate, for the fabrication of light emitting diode. In order to investigate the structural, morphological, optical and electrical properties of ZnO nanostructures and light emitting diode (LEDs) we used different experimental and characterization techniques.

3.1 Growth methods of ZnO nanorods

3.1.1 Low temperature chemical growth

To grow ZnO nanostructures there are several different chemical growth methods but in recent years wet chemical methods have received more attention and are now commonly used method for the growth of ZnO nanostructures. One of its major advantages is that the growth temperature is < 90 oC and with such low temperature we can grow ZnO nanorods on very cheap substrates e.g. plastic, paper and glass. On these substrates we can use p-type polymers when we want to produce a pn-hybrid junctions with ZnO nanorods, as the ZnO nanorods are n-type.

3.1.2 Growth of ZnO nanorods on PEDOT:PSS coated flexible plastic substrate

To grow ZnO nanorods on Poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) PEDOT:PSS coated flexible plastic substrate, the substrate was cleaned in ultrasonic cleaner in acetone, isopropyl alcohol and de-ionised water sequentially. We cover small portion of the PEDOT:PSS which later work as an electrode then the substrate was spin coated with Polyfluorene(PFO) and backed at 100 oC for 5 minutes. The PFO was mixed at a 1:1 molar ratio in toluene solvent. This gave a PFO film with 20 nm thickness. To improve the ZnO nanorods growth quality, distribution and density the substrate was coated with a seed layer and baked at 100 oC for 10 min [1]. The sample was placed in the solution containing a mixture of 0.01M zinc nitrate hexa hydrate (Zn (NO3)2.6H2O) and 0.01M

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20 3.1.3 Growth of ZnO nanorods on paper substrate

For the growth of ZnO nanorods on paper we have to overcome the rough surface of paper because the paper is synthesized from cotton/pulp contains cellulose due to that it has very rough surface. In order to overcome the surface roughness the surface is modified by evaporating Cr/Ag on paper. This Cr/Ag used as a electrode and also reduces the surface roughness to enhance the alignment and uniformity of the ZnO NRs on the paper substrate [2]. A hole transporting layer of Poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) PEDOT: PSS was spin-coated onto the paper substrates and baked at 80 oC for 5 minutes to form a uniform film. A layer of Polyfluorene (PFO) was spin-coated on the PEDOT: PSS film and was backed for 5 mints at 80 oC. To grow ZnO nanorods we used the hydrothermal growth technique. In this method 0.01M zinc nitrate hexa hydrate (Zn (NO3).6H2O) was mixed with 0.01M Hexamethyl tetra-mine (HMT) (C6H12N4). The sample

was then placed in the solution and was heated at 80 oC for 6 hours. 3.1.4 Growth of ZnO nanorods on glass substrate

In order to clean the glass substrate it was washed in ultrasonic cleaner using acetone, isopropyle and de-ionised water for 10 min. A hole transporting layer of Poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) PEDOT: PSS was spin-coated onto the glass substrate for 5 minutes and baked at 80 oC to form a uniform film. On top of the PEDOT: PSS film a layer of Polyfluorene (PFO) was spin-coated and backed at 80 oC. To grow ZnO nanorods we used the hydrothermal growth method. In this method we mixed 0.01M zinc nitrate hexa hydrate (Zn (NO3).6H2O) with 0.01M Hexamethyl tetra-mine (HMT)

(C6H12N4).The sample was then placed in the solution and was heated at 80 oC for 6 hours.

During the process the ZnO nanorods were formed on the substrate.

3.2 Growth of ZnO nanorods by high temperature method or the

vapor-liquid-soild (VLS) mechanism

3.2.1 Cleaning of the substrate

For the growth of ZnO nanorods by the chemical vapour deposition we used 4H-p-SiC substrate. In this growth method before any growth can take place, all samples need to prepared. For the growth of ZnO nanorods by the vapor-liquid-soild (VLS) mechanism cleaning of the samples is very important because if the substrate is clean then one can obtain

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good, reliable and reproduceible results. In order to clean the substrate we use a standard cleaning process RCA -1. During the RCA-1 cleaning insoluble organic residues are removed from the wafer surface [3].This was followed by RCA-2 to remove any metallic species. 3.2.2 Evaporation and cutting of the substrate

The vapour-liquid-solid (VLS) technique is a catalytic growth so the substrates need to have a catalyst metal on the surface. In this study we used a very thin film (1-5 nm) of Au on p-SiC. The Au was deposit by a standard evaporation technique. After the evaporation of Au we used a diamond saw to cut the substrate into small pieces Since for the boat used during the VLS growth a substrate of 6x8 mm2 is suitable [3].

3.2.3 VLS growth method

We adopted the vapour-liquid-solid technique for the growth of ZnO NRs [3]. The VLS mechanism was first time mentioned by R.S. Wagner and W.C Ellis in 1964 [4]. In recent years the growth of various nanostructures was achieved by the VLS processes. In this method a thin layer of Au nano-particles were deposited on the substrate which is use as a catalyst and heated to form a liquid droplet of the catalyst and the nanostructures on the surface. The main principle is still the same as in Wagner and Ellis paper [4].

3.2.4 Growth of ZnO nanorods on 4H-P-SiC substrate by the VLS methods

We adopted the vapour-liquid-solid (VLS) technique for the growth of ZnO NRs on 4H-P-SiC [3]. The substrate coated with Au which is used as a catalyst is placed in a horizontal tube furnace. The substrate is placed within a certain distance from the boat containing the reactant, i.e. the ZnO. A mixture of 1:1 ZnO:C powder with respect to weight, was placed into the boat and the substrate was placed above the mixture, with the catalyst pointing towards to ZnO:C powder. The boat was placed in the middle of the quartz tube.

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22

Figure 3.1: The VLS growth method [5].

A constant Ar gas flow of 80 sccm was applied for 5-10 min to maintain a stable environment inside the tube. Then we increased the temperature from room temperature to around 890 oC. The temperature was varied between 875-910 oC and the growth time varied from 5 min to 1 hour. At high temperature the ZnO is reduced to Zn vapor. The reaction is given by:

ZnO (s) + Cgraphite (s) → Zn (g) + CO (g)

ZnO (s) + CO (g) → Zn (g) + CO2 (g)

Figure 3.2: The tube furnace setup [5].

We commonly used argon as an inert gas; the reaction products are transported to the substrate, where the Zn vapour forms a liquid alloy with the catalyst. The formation of ZnO is assumed to be a result of a reaction between Zn and CO/CO2.

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3.3 Bottom contact

ZnO-based semiconductors are of great technological importance for the fabrication of optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes. Further improvement of the LED performance can be achieved through the enhancement of the external quantum efficiency. In this regard, high quality ohmic electrodes having low contact resistance and high transmittance (or reflectivity), along with thermal stability, must be developed because ohmic contacts play a very important role in the performance of LEDs. An ohmic contact on a semiconductor device has been prepared so that the current-voltage (I-V) curve of the device is linear and symmetric [6].

In this work, different metals were used to form the bottom contacts to the 4H-p-SiC and p-polymer. A Ni/Al alloys with a thickness of Ni and Al layer of 50 nm and 400 nm, respectively were used to form ohmic contacts on 4H-SiC. The samples were annealed in nitrogen and argon (N2/Ar) gass for 2 minutes at 500 oC. This contact gives specific contact

resistance of 7.9x 10-6 Ω-cm-2 [7].

3.4 Spin coating of photo resist, plasma etching and top contact deposition

An insulating photo-resist layer was spin coated on the ZnO nanorods to fill the gap between them and to isolate electrical contacts from reaching the p-type substrate. After the spin coating of the photo-resist, using oxygen plasma ion etching top of the ZnO nanorods were exposed. Al metal was used to form the ohmic contact to the ZnO.

3.5 Scanning electron microscope

The nanostructures grown by both the ACG and the VLS methods were characterized by a scanning electron microscope (SEM). The result show that the ZnO NRs were grown vertically aligned. Using the SEM it is possible to determine if any growth has been taken place because the SEM gives information on the morphology of the surface of the sample. However from the SEM images we cannot say that the obtain nanostructures actually consists of ZnO. From the SEM images we can find the diameter, length shape and growth rate of the nanostructures. Figure 3.3 shows the schematic diagram of the SEM apparatus. In our experiment a JEOL JSM-6301F scanning electron microscope was used.

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24

Figure 3.3: A schematic diagram of a SEM [8] .

The pressure inside the chamber is about 10-6 mbar and the gun voltage used is 12 KV. We can achieve maximum resolution of upto 10nm. In order to take the images through the SEM we have no need to do a lot of preparation on the sample.

3.6 Photoluminescence (PL)

In order to probe the electronic structure of materials photoluminescence spectroscopy is a common nondestructive method. In this method when light falls onto the sample some portion of the light is absorbed by the sample and imparts rest of the energy into the material. This process is called photo-excitation. During the process of photo-excitation excess energy can be dissipated by the sample through the emission of light, or luminescence and this luminescence is called photoluminescence (PL). This technique has been used to determine the bandgap and impurity levels, we can also use it to investigate the intrinsic and the extrinsic properties of semiconductors.

In PL light is used to create electron hole pairs in the semiconductor and the electrons will accumulate in the conduction band minimum and the hole in the valence band maximum.

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When these electrons return to their equilibrium states, they will recombine and the excess energy is released from the sample, if the light is emitted from the sample then it is called a radiative process but if there is no emission of light then it is a nonradiative process. The emitted light is detected with a detector. The amount of energy emitted during the photoluminescence process is equal to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state.

Figure 3.4: A schematic PL experimental setup [3].

A schematic diagram of our PL setup is given in the figure 3.3; in this a laser line from an Ar+ laser was used as the excitation source. In order to disperse and detect the emission from ZnO a double grating monochromator and photomultiplier detector were used [3].

3.6.1 Uses of photoluminescense

(a) Band gap determination

PL is use to determine the bandgap of semiconductors. In semiconductors the radiative transition between states in the conduction and the valence bands, with the energy difference being known as the band gap.

(b) Impurity levels and defect detection

Radiative transitions in semiconductors also involve localized defect levels and the energy associated with these levels during the photoluminescence measurement can

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26

identify specific defects, and also with the relative intensity of photoluminescence we can determine their concentration.

(c) Recombination mechanisms

When an electron returns to its equilibrium position, a process known as "recombination," it can involve both radiative and nonradiative processes. In semiconductors there are several possible recombinations mechanisms such as band-to-band transition and donor-acceptor pair recombination. Direct bandgap semiconductor materials have high probability for radiative recombination of electron-hole pair. Such semiconductors have strong bandgap radiation and can be used for LEDs.

(d) Band-to-band transition

Energy vs. crystal momentum for a semiconductor with a direct band gap, show (figure 3.4) that an electron can shift from the lowest-energy state in the conduction band (green) to the highest-energy state in the valence band (red) without a change in crystal momentum. Band to band recombination dominates at higher temperatures where all shallow defects are ionized. The carriers can be frozen on defects at lower temperature. In case of free to bound transition a free carrier recombine with a bound carrier [9].

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(e) Excitonic emission in ZnO

In ZnO exciton recombination is more efficient than band-to-band transitions and ZnO has high exciton binding energy. At room temperature the excitons are thermally stable and accordingly ZnO has a strong near band edge emission. At room temperature the free excitons (FE) dominate the excitonic emission and FE emission peak is centered around 380 nm (3.37 eV). At low temperature the bound excitons dominates the excitonic emission. (f) Material Quality

We can determine the materials quality and device performance in the nonradiative process because in the nonradiative process there are localized defect levels. With the amount of radiative recombination we can identify the material quality.

3.7 Cathodoluminesence (CL)

High energy electron bombardment of materials can cause emission of light, the process is known as cathodoluminescence (CL). In order to get the detailed emission information and to explain the origin of specific emission from specific small area, even from individual nanostructures, cathodoluminescence spectroscopy is performed, which records luminescence after creating electron–hole pairs by high energy electron bombardment. During the CL measurement the sample was on an evacuated CL-stage to an optical microscope. The CL-stage attachment uses a cathode gun to bombard a sample with a beam of high-energy electrons.The resulting luminescence in minerals allows us to see textures and compositional variations that are not otherwise evident using light microscopy.

.

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28 3.7.1 Type of optical-CL

Optical-CL attachements can be categorized in two types: 1. Cold-cathode CL

2. Hot-cathode CL Cold-cathode CL

The commonly used optical-CL system is called a cold-cathode CL. This system is attached to a microscope so one can examine the sample optically with the microscope and with CL in the same area. In a cold-cathode CL system the electron beam is generated by a discharge that takes place between the cathode at a negative high voltage and an anode at ground potential in an ionized gas at a moderate vacuum of ~10-2 Torr. The result is relatively low-intensity CL in most CL-active silicate minerals. Because some of the electrical charges at the discharge may reach the sample and neutralize the static charge built up, it is not necessary to coat the sample with a conductive coating. The CL response in the sample can be viewed through the objective lens of the microscope or the image can be recorded with high-speed film or with a digital camera (Figure 2) [10].

Figure 3.7: Top and cross-sectional views of a luminoscope optical-CL system .Modified after Marshall (1988) [10].

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Hot-cathode CL

A hot-CL instrument generates the electrons by a heated filament and the electrons are directed at lower voltage toward the anode. The vacuum is generally lower (<10

-5_{Torr) and the accelerating potential is about 14 keV. This stage has a more stable beam and}

has greater CL intensity. There are several non-commercial versions of the hot-CL instrument. These instruments have higher sensitivity and the ability to detect short-lived luminescence in minerals.

3.8 Current-voltage characteristics

The current versus voltage (I vs V) measurements were performed for the fabricated LEDs by using semiconductor parameter analyzer (SPA). The I-V curves provide different information about the information about the LEDs such as its turn-on voltage, break-down voltage forward and reverse currents, ideality factor, and parallel and series resistance of the fabricated LEDs.

3.9 Electroluminescence (EL)

Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to the passage of a strong electric field. In this phenomenon as a result of radiative recombination of electrons and holes in a material, usually in a semiconductor we get an emission spectrum. In semiconductors electroluminescent devices such as LEDs, Prior to recombination, electrons and holes may be separated either by doping the material to form a p-n junction or through excitation by impact of high-energy electrons accelerated by a strong electric field.

In this thesis the electroluminescence (EL) from n-ZnO nanostructures/p-4H-SiC, p-polymers LEDs is investigated. The electroluminescence (EL) measurements were performed at room temperature. The fabricated LEDs were illuminated by applying a forward biasing across the electrodes by using kiethely 2400 apparatus with the help of metal probes.

3.10 Deep level transient spectroscopy (DLTS)

The crystal quality of a semiconductor material is affected by defects such as dislocations, interstitial, vacancies and antisites. The impurity atoms might get into the crystal during the growth or by diffusion of different kinds of materials, metal deposition, chemical

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30

treatments, and annealing and ion bombardment. Due to the presence of all these defects and impurities, some localized energy levels called deep levels appear inside the band gap. All defects are not equally important. Their properties like the position in the band gap or the optical or electrical capture cross section that determine which one the critical defects.

Several techniques were proposed to get information about these deep levels but the ingenuous idea of using the change of capacitance under bias conditions due to the refilling of deep levels was proposed in the sixties [11]. In 1974 D. V. Lang proposed the deep level transient spectroscopy (DLTS) technique [12] which is a high frequency (MHz range) junction capacitance versatile technique for the determination of the parameters associated with traps including density, thermal cross section, energy level and spatial profile. By monitoring the capacitance transients produced by pulsing the semiconductor junction at different temperatures, a spectrum is generated which exhibits a peak for each deep level. The height of the peak is proportional to the trap density, its sign allows one to distinguish between the majority and the minority traps and the position on the temperature axis leads to the determination of the fundamental parameters governing the thermal emission and capture (activation energy and cross section). The DLTS is a quantitative, electrical-characterization method, which is used to survey deep levels in the band gap of semiconductor materials. Each deep level in the band gap results in a peak in the DLTS spectrum and a spectrum usually contains several peaks corresponding to different defects. From the peak position, the apparent energy position and apparent carrier capture cross section can be extracted.

3.11 Color rendering index (CRI)

The color rendering index (CRI) is a quantitative measure of the ability of a light source to reproduce the true colors of the objects in comparison with a natural light source. The color rendering index (CRI) is one of the most important parameter when we use LEDs for indoor and outdoor lighting applications and in that case the CRI of LEDs should be high.

The fabricated LEDs were illuminated by applying a forward biasing injection current across the electrodes by using Kiethely 2400 apparatus with the help of metal probes. The EL spectra of the emitted light from the fabricated LEDs were measured by a photomultiplier detector. The value of the color rendering index (CRI) and the correlated color temperature (CCT) were extracted from the EL spectra of fabricated LEDs by using computer software.

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References:

[1] M. Vafaee and H. Youzbashizade, Mat. Sci. Forum 553, 252 (2007).

[2] A. Manekkathodi, M. Y. Lu, C. W. Wang and L. J. Chen, Adv. Mater. 22, 4059 (2010). [3] P. Klason P 2008 Ph.D. Thesis, Department of Physics, University of Gothenburg,

ISBN 978-91-628-7492-6

[4] R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964). [5] http://www.mtixtl.com/xtlflyers/vlspapers/JustinHwaReport.pdf [6] http://en.wikipedia.org/wiki/Ohmic_contact

[7] J. Vang, M. Lazar, P. Brosselard, C. Raynaud, P. Cremillieu, J. L. Leclercg, J. M. Bluet, S. Scharnholz, and D. Planson, Superlattices and Microstructures 40, 626 (2006). [8] http://www.britannica.com/EBchecked/media/110970/Scanning-electron-microscope [9] http://en.wikipedia.org/wiki/Direct_and_indirect_band_gaps

[10] http://serc.carleton.edu/research_education/geochemsheets/opticalcl [11] R. Williams, J. Appl.Phys. 37, 3411 (1966).

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Fabrication and Characterization of ZnO Nanorods Based Intrinsic White Light Emitting Diodes (LEDs) Nargis Bano

Fabrication and Characterization of ZnO Nanorods Based Intrinsic

White Light Emitting Diodes (LEDs)

Nargis Bano

Fabrication and Characterization of ZnO Nanorods Based Intrinsic White

Light Emitting Diodes

Nargis Bano

Abstract:

Acknowledgement

Table of Contents

Chapter 1

Introduction

Chapter 2

Zinc oxide material

2.1 Basic properties of ZnO

2.2 Crystal structure of ZnO

2.3 Physical properties of ZnO at RT

2.4 Electrical properties of ZnO as compared to GaN

2.5 Properties and device applications

2.6 Optical properties of ZnO

2.7 ZnO nanostructures

2.8 Native point defects in ZnO

2.9 Classification of defects

2.10 Exciton

2.11 Recombination

Chapter 3

Growth and characterization of ZnO nanorods based LEDs

3.1

Growth methods of ZnO nanorods

3.2 Growth of ZnO nanorods by high temperature method or the

vapor-liquid-soild (VLS) mechanism

3.3 Bottom contact

3.4 Spin coating of photo resist, plasma etching and top contact deposition

3.5 Scanning electron microscope

3.6 Photoluminescence (PL)

3.7 Cathodoluminesence (CL)

3.8 Current-voltage characteristics

3.9 Electroluminescence (EL)

3.10 Deep level transient spectroscopy (DLTS)

3.11 Color rendering index (CRI)